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The present paper contrasts the relative efficacies of the traditional mean square or variance, and a time-series statistic, the mean square successive difference, as descriptors of cyclic heart rate variability. Estimates of fluctuation in the R-R interval during the prestimulus and cold pressor stimulus periods were obtained for two subject samples. The first sample was composed of individuals having bilateral differences in the diameter of their pupils, a condition known as anisocoria, while the pupils of the Ss in the second sample were of equal diameter. A group sinusoidal rhythm in the mean square successive difference of R-R interval variation was observed during the cold pressor stimulus for the anisocoric sample. However, the rhythm was obscured when the traditional mean square statistic was utilized to describe the same R-R interval variation. No rhythms were identified for the non-anisocoric sample. The mean square successive difference statistic would appear to provide a method for detecting cyclic autonomic variability patterns otherwise obscured by traditional analysis. [ABSTRACT FROM AUTHOR]